CS2100-CP

DS840F1

13

5. APPLICATIONS

5.1

Timing Reference Clock Input

The low jitter timing reference clock (RefClk) can be provided by either an external reference clock or an

external crystal in conjunction with the internal oscillator. In order to maintain a stable and low-jitter PLL out-

put the timing reference clock must also be stable and low-jitter; the quality of the timing reference clock

directly affects the performance of the PLL and hence the quality of the PLL output.

5.1.1

Internal Timing Reference Clock Divider

The Internal Timing Reference Clock (SysClk) has a smaller maximum frequency than what is allowed on

the XTI/REF_CLK pin. The CS2100 supports the wider external frequency range by offering an internal

divider for RefClk. The RefClkDiv[1:0] bits should be set such that SysClk, the divided RefClk, then falls

within the valid range as indicated in “AC Electrical Characteristics” on page 7.

It should be noted that the maximum allowable input frequency of the XTI/REF_CLK pin is dependent

upon its configuration as either a crystal connection or external clock input. See the “AC Electrical Char-

acteristics” on page 7 for more details.

For the lowest possible output jitter, attention should be paid to the absolute frequency of the Timing Ref-

erence Clock relative to the PLL Output frequency (CLK_OUT). To minimize output jitter, the Timing Ref-

It should be noted that there will be a jitter null at the zero point when N = 32 (not shown in Figure 10). An

example of how to determine the range of RefClk frequencies around 12 MHz to be used in order to

achieve the lowest jitter PLL output at a frequency of 12.288 MHz is as follows:

where:

and

Referenced Control

Register Location

RefClkDiv[1:0] ....................... “Reference Clock Input Divider (RefClkDiv[1:0])” on page 28

Figure 9. Internal Timing Reference Clock Divider

N

Internal Timing

Reference Clock

PLL Output

Fractional-N

Frequency

Synthesizer

Timing Reference

Clock Divider

XTI/REF_CLK

RefClkDiv[1:0]

8 MHz < SysClk < 18.75 MHz

8 MHz < RefClk <

Timing Reference Clock

50 MHz (XTI)

75 MHz (REF_CLK)

-80

-60

-40

-20

0

20

40

60

80

20

40

60

80

100

120

140

160

180

Normalized REF__CLK Frequency (kHz)

CLK__OUT Jitter

-15 kHz

+15 kHz

CLK__OUT

f

*32/N

Figure 10. REF_CLK Frequency vs a Fixed CLK_OUT

f

L

f

RefClk

f

H

≤≤

f

L

f

CLK_OUT

31

32

------15kHz

+

×

=

12.288MHz

0.96875

15kHz

+

×

=

11.919MHz

=

f

H

f

CLK_OUT

32

32

------15kHz

–

×

=

12.288MHz

115kHz

+

×

=

12.273MHz

=